1. Field of the Invention
The present invention relates to a magnetoresistive device adapted to read the magnetic field intensity of magnetic recording media or the like as signals, and a thin-film magnetic head comprising that magnetoresistive device as well as a head gimbal assembly and a magnetic disk system, one each including that thin-film magnetic head.
2. Explanation of the Prior Art
A thin-film magnetic head is built up of a reproducing head having a reading magnetoresistive device (MR device) and a recording head having a writing induction type electromagnetic device.
A typical construction of part of the MR device includes a multilayer structure made up of an antiferromagnetic layer/first ferromagnetic layer/nonmagnetic intermediate layer/second ferromagnetic layer.
The first ferromagnetic layer is also called a fixed magnetization layer because its direction of magnetization remains fixed by the antiferromagnetic layer adjacent to it. By contrast, the second ferromagnetic layer is referred to as a free layer because its direction of magnetization changes by way of sensitive reaction with an external magnetic field change.
A lower electrode and shield layer and an upper electrode and shield layer (hereinafter often called the shield layers for short) are provided such that the aforesaid multilayer MR device is sandwiched between them from above and below, so that leaking magnetic fluxes from adjacent recorded data can be cut off.
The shield layers located above and below the multilayer MR device are each made of a soft magnetic thin film, and one each is generally formed in such a way as to have a closure domain structure with an easy axis of magnetization in the same direction as the track width direction of the MR device. Here, if the domain structures of the shield layers are not stably formed with variations per head, then there are also variations in the bias state applied to the MR device (especially a bias applied to the free layer), which may otherwise give rise to a problem of offering an obstacle to improvements in production yields.
In association with an increasing magnetic recording density, it is increasingly required for the MR device to be much narrower than ever before in terms of track width and read gap. As the read gap grows narrower, it causes the distance between the shield layers and the free layer forming the MR device portion to decrease drastically, letting the shield layers have strong influences on the bias state of the MR device portion. Specifically, if there is a change in the domain structures of the shield layers due to an external magnetic field, it could bring about an output fluctuation of the MR device. That is, the resistance to an external magnetic field (magnetic field resistance) could get worse.
In conjunction with such problems, JP(A)2007-242140 discloses a technique for configuring the planar shape of a shield portion in a reproducing head into an annular one to control the domain structure of the shield portion thereby reducing output fluctuations of the reproducing head under the influences of the shields and, hence, obtaining stabilized outputs.
As far as what is set forth in the aforesaid publication is concerned, however, control of the domain structure of the shield portion would be still less than satisfactory. Thus, there is a mounting demand for the development of a novel technique for holding back output fluctuations of the reproducing head under the influences of shields and, hence, obtaining stabilized outputs.
By the way, a further slimming-down of the magnetoresistive device is now in great need so as to meet recent demands for ultra-high recording density. Such being the case, there is a novel GMR device structure put forward, which has a basic structure comprising a simple triple-layer arrangement of ferromagnetic layer (free layer)/nonmagnetic intermediate layer/ferromagnetic layer (free layer), as set forth typically in U.S. Pat. No. 7,019,371B2 or U.S. Pat. No. 7,035,062B1.
For the sake of convenience, such structure is here called the dual free layer (DFL) device structure. In the DFL device structure, the two ferromagnetic layers are exchange coupled together such that their magnetizations are antiparallel with each other. And under the action of a bias magnetic field given out of a magnet located in a depth position opposite to the ABS corresponding to the surface of the device facing a medium, there is an initial state created in which the magnetizations of two magnetic layers (free layers) are inclined about 45° with respect to the track width direction. Upon detection of a signal magnetic field from the medium in the initial state of the device, the directions of magnetization of the two magnetic layers change as if scissors cut paper, with the result that there is a change in the resistance value of the device.
When such DFL device structure is applied to the so-called TMR or CPP-GMR device, it is possible to make the gap (read gap length) between the upper and lower shield layers much narrower as compared with a conventional, ordinary spin valve type CPP-GMR device. Specifically, it is possible to make do without the aforesaid antiferro-magnetic layer that is needed for the ordinary spin valve type CPP-GMR device as well as the ferromagnetic layers of the aforesaid “synthetic pinned structure”. As a result, the “read gap layer” that has been said to be 30 nm at the very most can be reduced down to 20 nm or less.
However, the proposals made so far are all directed to (1) the technology that underlies exchange coupling of two ferromagnetic layers via the nonmagnetic layer, and (2) the technology that makes use of two antiferromagnetic layers to place the magnetizations of two ferromagnetic layers (free layers) in an anti parallel state. For this reason, although depending on what material is used for the nonmagnetic layer, it is impossible to obtain any strong exchange coupling between two ferromagnetic layers, or there is a problem that the purpose of reducing the “read gap length” (the gap between the upper and lower shield layers)) is only achievable at the cost of practical advantages.
The situations being like this, Applicant has already filed U.S. Ser. No. 11/946,358 for the purpose of providing a novel magnetoresistive device that makes it possible to achieve an antiparallel magnetization state for two ferromagnetic layers (free layers) with simple structure yet without being restricted by the material and specific structure of an intermediate film interposed between the two ferromagnetic layers (free layers), that makes it possible to improve on linear recording densities by the adoption of a structure capable of making the “read gap length” (the gap between the upper and lower shield layers) narrow thereby meeting recent demands for ultra-high recording densities, and that makes it possible to obtain stable magnetoresistive effect changes so that much higher reliability is achievable.
The invention of this application has for its object the provision of a magnetoresistive device that makes further progresses in the art of U.S. Ser. No. 11/946,358 already filed by Applicant, thereby stabilizing control of the domains of shield layers, improving on resistance to an external magnetic field, and improving more on the reliability of device operation.